TRex
Here's my weather recollections from the various marathons that I used to target.
April: Drizzle 50%. Sunburn 30%. Frost 10% Windy 10%. Hissing down 20% chance.
July: Sunburn 10% Heat 40%, Drizzle 20%. Wind 10%, Shower 10% Flood10%
Sept/Oct: Sunburn 0%. Drizzle 30% Single figure cold 40%, Wind 10% Gale 10% Fog 5% Rain 5% A really nice day 40%.
January: Frost 10%, Snow 1%, Ice 2%, Flood 2%, Sunburn 0%, drizzle 15%, Wind 10%, Rain 10%, A really nice day 50%.
Pedants will notice that the percentages don't add up to 100, as some options are not mutually exclusive. The irony is that I've had better weather in winter than summer. Most sunburn is in April, worst floods July, worst gales October, and January has produced a beautiful swatch of glorious blue sky days to remember. Run it for fun, as there are flatter courses out there, but this one really sets you up for April.